The read channel in a mass storage system “demodulates” data read from a magnetic medium, such as a hard disk, and converts the demodulated data into a form (e.g., digital data) which can be utilized by, for example, a computer.
The data is stored on a disk as symbols, the data having been encoded using a modulation code (e.g., a run-length limited code) and, typically, an error correction code (e.g., a Reed-Solomon and/or Low Density Parity Check) applied prior to encoding with the modulation code, to increase the reliability of the mass storage system. As will be explained in more detail below, the symbols are preferably recorded on the disk with uniform density to strive for the maximum data capacity that the disk can hold. One consequence of using uniform symbol density is that the rate at which the data symbols are read from the disk depends upon which track is being read since the disk spins at a substantially constant rate. In a typical mass storage system, the symbol rate can vary by a factor of two or more between the innermost and outermost tracks.
A typical prior art read channel comprises an analog front-end (AFE) and a digital back end (DBE). The AFE amplifies, filters, and, in response to a clock, digitizes the signals from a read head positioned over the disk, while DBE, also in response to the clock, processes the digitized analog samples from the AFE to recover the stored data. The frequency of the clock is related to the incoming symbol rate. A controller, knowing what track the read head is reading, sets the clock frequency. Generally, all of the circuitry in the read channel is designed to process in real-time the maximum symbol (data) rate that can be read from the disk.
For read channels capable of reading up to many hundreds of megabits per second of data from the disk, all of the circuitry in the read channel, particularly the DBE, is usually implemented in a high-speed, small-geometry, power-hungry, complementary metal-oxide-semiconductor process technology (e.g., 65 nm CMOS). The power consumed by the digital logic in the read channel is highest when the read channel is reading data from the outermost track. The amount of power needed by the read channel, particularly when reading the outermost tracks, might be too much for portable or other low-power applications.